1. Field of the Invention
The present invention relates to an optical scanning device, an optical scanning method, and an image forming apparatus.
2. Description of the Related Art
FIG. 17 is an example of a configuration of a conventional image forming apparatus (writing optical system). The conventional image forming apparatus is, for example, a laser printer or a digital copy machine. The conventional image forming apparatus performs electrophotographic process.
A laser diode unit 1001 (light source unit) emits a laser beam, and the emitted laser beam is deflected by a polygon mirror 1002 that is rotating. The deflected laser beam passes through a scanning lens (fθ lens) 1003, and illuminates a photosensitive element 1004 that is a medium to be scanned, and thereby an optical spot is formed on the photosensitive element 1004. Thus, an electrostatic latent image is formed onto the photosensitive element 1004 by projecting the laser beam onto the photosensitive element 1004. A phase synchronizing circuit 1009 synchronizes a phase of a modulation signal generated by a clock generating circuit 1008 with an output signal from a photodetector 1005. The photodetector 1005 detects a laser beam that is emitted from a laser diode and deflected by the polygon mirror 1002. Specifically, the phase synchronizing circuit 1009 generates an image clock (pixel clock) that is phase-synchronized with an output signal from the photodetector 1005 each time one line of an image is scanned, and supplies the generated image clock to an image processing unit 1006 and a laser driving circuit 1007. The laser diode unit 1001 controls light emission time of the laser diode through the laser driving circuit 1007 in accordance with image data generated by the image processing unit 1006 and the image clock generated by the phase synchronizing circuit 1009. Thus, it is possible to control forming of an electrostatic latent image on the photosensitive element 1004.
In recent years, faster printing speed (image forming speed) and higher image quality have been required, and improvement has been achieved in a rotational speed of a polygon motor serving as a deflector, and a frequency of a pixel clock used as a reference clock for modulating a laser beam from a light source. However, the rotational speed of the polygon motor and the frequency of the pixel clock cannot be further improved by a conventional technology.
In a conventional technology, a multi-beam optical scanning method has been developed in which a plurality of light sources is used. The multi-beam optical scanning method makes it possible to increase the number of light beams to be deflected by a deflector and projected onto a target surface at the same time. Therefore, it is possible to reduce the rotational speed of the polygon motor and the frequency of the pixel clock. Thus, optical scanning and image forming can be performed at a high speed and in a stable manner.
A light source used for the multi-beam optical scanning is configured by, for example, combining laser chips each emitting a single beam, or employing a laser diode (LD) array in which a plurality of light emitting elements is incorporated into one laser chip.
A laser diode included in the LD array has been widely used as a light source in a laser printer or the like, because such a laser diode is extremely small size, and a light beam from the laser diode can be directly modulated with a driving current at a high speed. However, the relation between the drive current and light output from the laser diode depends on a temperature, and therefore it is difficult to set a light intensity of the laser diode to a desired value. In a case of a surface-emitting laser, because a plurality of light sources is arranged on the same chip, the interval between the light sources becomes short. As a result, the surface-emitting laser is remarkably affected by a change in temperature due to switch ON/OFF of the light sources and temperature crosstalk. Therefore, it is likely to cause fluctuation in light intensity.
For example, Japanese Patent Application Laid-open No. 2001-272615 discloses a conventional optical scanning apparatus in which a plurality of light sources is arranged two-dimensionally, and a plurality of light beams from the light sources are deflected by a deflector, and projected onto a medium to be scanned. In the optical scanning apparatus, light emitting points are arranged at a maximum density while crosstalk due to heat that occurs between the light emitting points is prevented.
Japanese Patent Application Laid-open No. 2003-72135 discloses a conventional image forming apparatus that employs a surface-emitting laser. The image forming apparatus implements a method of controlling a pixel of an electrostatic latent image by changing light emission intensity of a chip for each pixel, and controlling light emission time.
Japanese Patent Application Laid-open No. 2001-350111 discloses a conventional scanning apparatus that employs a surface-emitting laser. In the scanning apparatus, light sources are arranged in a prescribed manner, so that thermal crosstalk can be prevented, and a density of recorded images can be improved.
However, a problem occurs when images are printed on both surfaces of a sheet. Specifically, the sheet shrinks due to heat applied when an image is printed on one surface (e.g., the front surface) of the sheet. If an image is printed on the other surface (e.g., the back surface) of the shrunk sheet, the image printed on the back surface is relatively larger than the image printed on the front surface due to the shrinking of the paper in printing of the image on the front surface.